The present invention relates generally to devices used for the dissipation of hydraulic or kinetic energy, and-more particularly to a spillway structure constructed from a plurality of pre-formed building blocks which are specifically shaped, dimensioned and arranged so as to induce the maximum amount of turbulence in water flowing thereover, thus minimizing the amount of kinetic energy in the water mass as it drops to the toe of the spillway.
As is well known in the field of hydraulic engineering, there is an ongoing need to inhibit erosion caused by rivers, streams and other waterways both natural and man made which occurs at locations where there is a change in grade. Well known in the prior art are various types of hydraulic energy dissipation devices which are commonly referred to under the collective term xe2x80x9cenergy dissipatorsxe2x80x9d, and are used to provide erosion control protection by serving as, among other things, dam spillways, drop structures in natural streams or man made channels, and grade control structures in natural streams or man made channels. The significant resources devoted by many governmental agencies to protect civil structures such as canals, dams or other waterways constructed of earthen materials from erosion has resulted in the development of a relatively wide range of prior art energy dissipators and other erosion protection systems.
One category of prior art energy dissipator is adapted to develop a high velocity at the toe or bottom of the drop, with dissipation of the hydraulic or kinetic energy being accomplished by a hydraulic jump. These types of energy dissipators typically shorten the length of the hydraulic jump and subsequently reduce the distance of high velocities downstream of the toe which would otherwise cause scour. In the prior art, there are numerous designs of these types of energy dissipators currently in use, with perhaps the most common being a stilling basin which incorporates special appurtenances (i.e., chute block, end sills, baffles, etc.) which tend to stabilize the hydraulic jump and improve its performance.
Other types of prior art energy dissipators currently under research include stepped spillways, roller compacted concrete (RCC), gabions, riprap, baffle apron drops, geotextiles, and concrete block revetment systems. However, as will be discussed below, these prior art energy dissipators each possess certain deficiencies which detract from there overall utility.
Gabions are wire baskets which are filled with rock and anchored to slopes for erosion protection. Though gabions have been successfully used for building dams with gabidn spillway weirs, research has indicated that though they may perform well if anchored properly, they undergo considerable deformation under certain flow conditions. More particularly, it has been determined that structural deformation of gabions could occur for flows in excess of sixteen (16) CFS or velocities between fifteen (15) and seventeen (17) feet per second. For flows in excess of these parameters, additional strengthening is typically required for the gabions.
With regard to riprap, research is currently underway regarding the use of riprap as a means of reducing toe velocities using rock chutes. Thus far, this research has indicated that there are limiting factors associated with the structural stability of riprap on steep slopes subject to high flows which severely limits its utility. In particular, modeling has demonstrated that riprap scaled to represent a median twenty-four (24) inch diameter rock on a 3 to 1 slope was only able to withstand a scaled unit discharge of under twenty three (23) CFS per foot. Flows in excess of this value exhibited failure by materials being dislodged and transported down the slope (chute). At present, the difficulties associated with accurately predicting the behavior of riprap protection has mitigated against its recommended usage as protection from overtopping flows of any significant magnitude.
Though roller compacted concrete (RCC) has proven to be very effective in protecting against erosion, the protection imparted thereby is attributable to the thickness of the concrete overlay alone. Though the applications for roller compacted concrete are widespread, they rely on the strength of the material and the cover thickness to provide erosion protection. It has been determined that subjecting the materials to high velocity flow would likely degrade the protective system. Additionally, the installation techniques associated with roller compacted concrete are generally economical only for the placement of large qualities of material, and further require easy site access. Moreover, roller compacted concrete may significantly impact the surrounding environment.
Though baffle apron drops have been used successfully in canal design, the systems have not had extensive use in flood control applications and are susceptible to damage from debris. Additionally, testing of geotextiles has indicated that failure occurs at relatively high velocities, with such failure believed to be caused by poor anchoring or stretching of the material.
Concrete block revetment systems (articulating blocks) are generally cable-tied together and anchored to the embankment, with grass being used to cover over the voids. However, the use of concrete block revetment systems is largely limited to erosion protection, with most of these systems being designed to prevent river bank erosion. Thus far, two such systems have been tested and are in limited use for overtopping protection, but are considered to be unsuitable for high flow velocities due to the energy dissipation properties being minimal.
Step spillways have been in use for thousands of years, and are-currently experiencing a re-emergence. In this respect, the step spillway is currently under strong research with many hydraulic researchers believing that step spillways will be included with the more classical types of energy dissipators currently being used in erosion protection applications. The stepped spillway is a simple form of a rough channel wherein a stable rolling vortex is developed within each step. This rough channel does not allow the velocity down the drop to reach the velocity that would occur on a smooth spillway, with these reduced toe velocities having an effect on the stilling basin design at the toe. The stable rolling vortex created within each step of the stepped spillway as discharge flows down the drop dissipates a considerable amount of hydraulic or kinetic energy. However, although dissipating energy, the vortex also acts as a xe2x80x9ccushionxe2x80x9d for skimming flows. This particular hydraulic characteristic results in little lateral movement of the water or discharge as it flows down the drop and velocities that are basically two dimensional.
The shortcomings of the above-described prior art energy dissipators are overcome by the offset stepped spillway constructed in accordance with the present invention which dissipates energy at rates exceeding several magnitudes above any known prior art energy dissipation system. In this respect, the offset stepped spillway of the present invention has the ability to dissipate large amounts of kinetic energy on a continuous basis, and possesses several hydraulic characteristics which represent a significant departure from those associated with prior art stepped spillways. In particular, the offset steps and stacking pattern in the present spillway annihilates any semblance of a stable vortex at each step, and creates high gradient velocity zones. Additionally, the offset steps and stacking pattern creates a high lateral diffusing of velocities, and thus transforms two dimensional flow into three dimensional flow. Moreover, the offset steps and stacking pattern is believed to generate slight vortex rollers, with the offset steps creating a shear zone where there is a negative (upslope) velocity component coming in contact with the primary positive (downslope) velocity component. This negative-positive contact is believed to occur on the drop and interferes with the primary direction of flow which reduces the velocity in the primary direction and thus dissipates additional kinetic energy.
In accordance with the present invention, there is provided a spillway for use in a sloped embankment which defines a top and a toe. The present spillway is adapted to dissipate the hydraulic or kinetic energy of water which flows downwardly from the top of the embankment to the toe thereof in a primary flow direction.
The spillway of the present invention comprises a plurality of building blocks which are arranged in rows, with each row including multiple building blocks disposed in side-by-side relation to each other. The rows of building blocks are stacked upon each other in a shingle-like overlap such that the building blocks of each row are laterally offset or staggered relative to the building blocks of each adjacent row and a series of steps are defined thereby. The building blocks are dimensioned and shaped such that water cascading down the steps defined thereby is caused to flow in three dimensions. Such three-dimensional flow imparts velocity components to the falling water that act at generally right angles relative to the primary flow direction and generate turbulence (i.e., create a churning action) which dissipates the kinetic energy of the water. Each of the building blocks is preferably fabricated from concrete, and includes a tether extending therefrom for anchoring the same to the embankment.
In addition to the building blocks, the present spillway further comprises at least one, and preferably multiple toe plates which extend along the toe of the embankment in side-by-side relation to each other. The lowermost row of building blocks are partially positioned upon and supported by the toe plate(s). The spillway also includes at least one crest plate which extends along the top of the embankment. The crest plate itself is partially positioned upon and supported by the uppermost row of building blocks. Like the building blocks themselves, the toe and crest plates are each preferably fabricated from concrete.
In the present spillway, the building blocks preferably comprise full blocks and half blocks. In this respect, the outermost building blocks of every other row in the spillway comprise half blocks, with the remaining building blocks comprising full blocks. The full blocks themselves each comprise a base portion having a pair of finger portions extending therefrom in spaced relation to each other. The base and finger portions of each full block collectively form a three-sided slot having a back wall which is defined by the base portion and opposed sidewalls which are defined by respective ones of the finger portions. In certain embodiments of the present invention, the slot has a generally rectangular configuration, while in other embodiments the slot has a generally trapezoidal configuration or a dovetail configuration. The half blocks each comprise one-half of a full block, and include a base portion having a finger portion extending therefrom. The base and finger portions of each half block collectively form a two-sided notch having a back wall which is defined by the base portion and a sidewall which is defined by the finger portion.
In the present spillway, the building blocks (i.e., the full and half blocks) are arranged such that the distal ends of each adjacent or abutting pair of finger portions in a particular row extend to the back wall of the slot of a respective full block in the row immediately therebelow. The distal end of each finger portion in a particular row not abutting another finger portion extends to the back wall of the notch of a respective half block in the row immediately therebelow. In accordance with an alternative embodiment of the present invention, the finger portions of the full and half blocks may each include a generally conical indentation or other shaped indentation formed therein to assist in the generation of turbulence within the falling water.
In another alternative embodiment of the present invention, the base portion of each full block may include a pair of generally conical indentations formed therein, with the finger portions each including a generally conical indentation formed therein. If configured in this manner, the full blocks are arranged in the spillway such that the indentations in the finger portions of each of the full blocks in a particular row align with one of the indentations of respective ones of an adjacent pair of the full blocks in the row immediately therebelow. Each pair of the aligned indentations effectively creates additional energy dissipation properties. The half blocks used in conjunction with these particular full blocks each comprise one-half of a full block, and thus each include a single conical indentation formed in the base portion thereof.
In the present spillway, the desired dissipation of kinetic energy in the falling water is achieved by sizing the full and half blocks relative to the slope S of the embankment and to each other in a specific manner. More particularly, each of the full blocks is preferably sized to have a width W which is the product of a constant C and its height H, with the finger portions thereof each preferably having a length X which is equal to the product of the slope S and height H. The constant C is preferably from about 3 to 5, and most preferably about 4. In each of the full blocks, the distal ends of the finger portions each have a preferred width Z. The depth of the slot in each full block is equal to the lengths X of the finger portions thereof. In certain embodiments of the present invention, the width Y of the back wall of the slot of each full block is about two times the width Z and, in one embodiment, is about one-half the width W. In another embodiment of the present invention, the width Y of the back wall of the slot of each full block is equal to the difference between the width W and two times the width Z.
Since, as indicated above, each half block is configured as one-half a full block, each of the half blocks is preferably sized to have a width Wxe2x80x2 which is about one-half the width W of each full block, with the back wall of the notch of each half block preferably having a width Yxe2x80x2 which is about one-half the width Y of the back wall of the slot of each full block. Like each full block, each half block is of the height H, with the finger portion thereof being of the length X which is equal to the product of the slope S and height H. The distal end of the finger portion of each half block is also of the width Z.
Further in accordance with the present invention, there is provided a method for constructing a spillway usable in a sloped embankment defining a top and a toe and adapted to dissipate the kinetic energy of water flowing downwardly from the top of the embankment to the toe thereof in a primary flow direction. The method comprises the initial step of cutting a plurality of terraces into the embankment so as to define a series of steps which extend from the top to the toe. Thereafter, at least one toe plate is extended along the toe of the embankment. A plurality of pre-fabricated building blocks are then arranged upon the terraces in rows which are stacked upon each other in a shingle-like overlap. The arrangement is accomplished such that the building blocks of each row are offset relative to the building blocks of each adjacent row in a manner wherein water cascading down the building blocks is caused to flow in three dimensions so as to impart velocity components to the falling water that act at generally right angles relative to the primary flow direction and generate turbulence which dissipates the kinetic energy of the water. Additionally, the lowermost row of building blocks is partially positioned upon the toe plate, with each of the building blocks being anchored to the embankment through the use of a tether extending therefrom. Finally, a crest plate is partially positioned upon the uppermost row of building blocks.
The construction of the spillway may also proceed from the bottom of the embankment upward by backfilling against the building blocks to create the terrace effect. This process is repeated until the desired height of the spillway and embankment is obtained. Additionally, a spillway having the structural and functional attributes described above may be created through the use of field formed concrete which is cast in place rather than through the use of the building blocks.
Still further in accordance with the present invention, there is provided a method for dissipating the hydraulic or kinetic energy of falling water. The method comprises the steps of causing the water to flow forwardly along horizontally oriented primary flow direction axes, and causing the water to flow laterally along horizontally oriented secondary flow direction axes which extend at generally right angles relative to the horizontally oriented primary flow direction axes. The method comprises the further step of causing the water to flow downwardly along vertically oriented primary flow direction axes which extend at generally right angles relative to the horizontally oriented primary flow direction axes and the secondary flow direction axes. The steps of imparting three dimensional flow to the water are preferably accomplished through the use of a spillway comprising a plurality of building blocks arranged in rows which are stacked upon each other in a shingle-like overlap such that the building blocks of each row are offset relative to the building blocks of each adjacent row and a series of steps are defined thereby.